In one form of drop on demand non-contact fluid droplet applicator, for example an ink jet printer, fluid is supplied under pressure to an array of nozzle orifices arranged so that droplets ejected from the nozzle orifices are applied to a substrate to form the desired image. The substrate moves relative to the orifices and fluid is allowed to flow to the nozzles in the desired sequence under the control of a valve mechanism in the fluid flow lines serving each nozzle. Typically, the valve is a solenoid type valve in which a plunger carrying a sealing member moves in and out of engagement with the outlet to a chamber fed with ink under a pressure of from 0.1 to 2 bar from the ink reservoir.
With an ink jet printer the objective is to form sharply defined images on the paper composed of a number of droplets ejected from the nozzle array. However, it has been found that at least some of the droplets emitted by the nozzles when the printer is started after a rest period, for example a pause during printing or even when a particular nozzle has not been utilised for some reason during part of a print run, are not uniform or may not be emitted at all from the nozzle orifice.
We have found that the valve plunger will, over a period of time, tend to move forward under the influence of the return spring after it has nominally seated into its valve closed position and closed the orifice. This "creep" occurs due to the resilient nature of the material used for the sealing member and/or the valve seat. The effect of this creep is to increase the separation between the magnetic pole faces formed by the moveable plunger and the fixed components of the valve construction. The resilient nature of the material also allows the material of the sealing member to deform around the member bearing against and this may provide a measure of grip between the sealing members. We believe that both these factors contribute to variable droplet ejection after periods of rest.
We have devised a method for reducing this problem without the need to modify the design or construction of the valve.